The present invention relates to a novel photocurable organopolysiloxane composition or, more particularly, to a photocurable organopolysiloxane composition having unusually high photosensitivity when irradiated with light rich in ultraviolet to be converted into insoluble and infusible cured products with excellent properties such as a coating film on a substrate.
Needless to say, various types of organopolysiloxane compositions are used in a variety of application fields owing to their unique properties as a polymeric material. It is a common practice that these compositions are used as cured in a cured state such as paints and varnishes for coating various substrates or an anti-sticking layer in release papers.
Such a cured coating layer is obtained, of course, by first providing a layer of an uncured organopolysiloxane composition on the surface of the substrate and then curing the organopolysiloxane composition with formation of crosslinks between the molecules of the organopolysiloxane. The crosslink formation between the organopolysiloxane molecules takes place in several different ways and the most conventional way for obtaining crosslinking of organopolysiloxane molecules is the condensation reaction or addition reaction between functional groups of the molecules accelerated by heat.
The method of heat curing is simple in principle and widely used but it is not applicable when a high curing velocity is desired. In addition, the application of heat curing is largely limited when the substrate material has an insufficient heat resistance.
Alternatively, photochemical means for curing organopolysiloxane compositions have been proposed in which crosslinks are formed between the photosensitive functional groups of the organopolysiloxane when the composition is irradiated with light, especially, rich in ultraviolet. Various kinds of photosensitive groups as well as photosensitizers have been proposed. As an example, a photocurable organopolysiloxane composition is disclosed in Japanese Patent Disclosure 5061386 comprising a first organopolysiloxane containing mercapto groups, a second organopolysiloxane containing vinyl groups and a photosensitizer. Mercapto-containing photocurable compositions of this type are disadvantageous because of the unpleasant odor as well as the relatively poor durability of the releasability when they are used for providing an antisticking layer on release papers.
Another example of the photocurable organopolysiloxane composition disclosed in Japanese Patent Publication 53-2911 comprises an organopolysiloxane having acryloxy groups or methacryloxy groups as the photosensitive functional groups, a photosensitizer and, optionally, an organic solvent. One of the problems in the photocurable compositions of this type is that the surface of the photocured film of the composition is more or less tacky even after prolonged irradiation with light when photocuring is effected in air due to the influence of the atmospheric oxygen so that the irradiation should be performed in an atmosphere of nitrogen or other inert gases when complete removal of surface tackiness is desired.
In addition to the above described problems in the prior art photocurable organopolysiloxane compositions, further and further increased photosensitivity is eagerly desired as a matter of course for improving productivity.